Spread spectrum receiver with fast signal reacquisition

ABSTRACT

A spread spectrum receiver, such as a GPS car navigation system, uses multiple, parallel correlations of data in a wide capture window, permits seamless tracking and rapid reacquisition of satellite signals at speeds sufficient to permit position information updates during momentary views available of satellites in intersections. The received data is processed in 11 bit samples, for a total of 186 time segments per 1 msec code period, which are each Doppler shifted and multiplexed into 12 channels. The code correlations are performed 22 times per channel by shifting the 11 bit locally generated code by one bit each correlation. The chip rate of the incoming data is 2f 0  and the correlations are performed at 48f 0  providing a time magnification advantage of 24.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Application Ser.No. 60/013,514, filed Mar. 15, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to spread spectrum receivers and inparticular to GPS navigation systems such as those used in terrestrialnavigation for cars, trucks and other land vehicles.

2. Description of the Prior Art

Car navigation is conventionally performed using highway and street mapsaided, to some degree, by distance measurements from external sensorssuch as odometers. Improvements over the last 10 years in GlobalPositioning System, or GPS, satellite navigation receivers has spawnedseveral GPS car navigation systems.

Conventional GPS car navigation systems use the last known position ofthe vehicle, and the destination data, to compute a route data base,including route and turning data derived from a pre-existing map database. GPS receivers are conventionally operated with a minimum of 3 or 4satellites distributed across the visible sky in order to determine, orat least estimate, the four necessary unknowns including x_(user),y_(user) and z_(user) which provide three orthogonal coordinates tolocate the user as well as t_(user) which provides the requiredsatellite time. Techniques such as time or clock hold and altitude hold,in which the unknown time or altitude is assumed to remain predictablefrom a previously determined value, e.g. z_(est) and/or t_(est), havepermitted operation of GPS receivers with less than 4 satellites inview. In particular, terrestrial GPS receivers have been operated withas few as 2 satellites to provide a 2 dimensional position solutionusing both clock and altitude hold.

Because continuous reception from 4 GPS satellites is often difficult tomaintain in a car navigation environment, and known clock and altitudehold techniques can only permit operation with at least 2 satellites,known conventional car navigation systems have typically augmented theGPS position information with information from external sensors toprovide dead reckoning information. The dead reckoning information isoften provided by an inertial navigation system such as a gyroscope.

Augmenting GPS data with inertial navigation data has permitted the useof GPS car navigation even when less than 4 satellites are visible, suchas in tunnels and in urban situations between tall buildings. However,the resultant increased complexity and costs for such combined systemshave limited their acceptance.

Conventional GPS receivers use separate tracking channels for eachsatellite being tracked. Each tracking channel may be configured fromseparate hardware components, or by time division multiplexing of thehardware of a single tracking channel, for use with a plurality ofsatellites. In each tracking channel, the received signals areseparately doppler shifted to compensate for the relative motion of eachsatellite and then correlated with a locally generated, satellitespecific code.

During a mode conventionally called satellite signal acquisition,delayed versions of the locally generated code for the satellite beingacquired are correlated with the doppler rotated received signals tosynchronize the locally generated code with the code, as received forthat satellite, by determining which delay most accurately correlateswith the code being received. Once synchronization has been achieved fora particular satellite, that satellite channel progresses to a trackingmode in which the doppler rotated, received signal is continuouslycorrelated with the locally generated code for that satellite todetermine position information including pseudorange information. Duringtracking, conventional receivers also correlate the doppler shiftedreceived signal with one or more versions of the locally generated codeat different relative delays, such as one half C/A code chip width earlyand late relative to the synchronized or prompt version of the code.These early and late correlations are used to accurately maintain thesynchronization of prompt correlation.

When, after tracking has begun for a particular satellite, the satellitesignal has been lost so that the required timing of the locallygenerated code for synchronization is no longer accurately known,conventional receivers reenter the acquisition mode, or a limitedversion of this mode, to reacquire the satellite signals by multiplecorrelations to resynchronize the locally generated code with the codeas received. Once the locally generated code has been resynchronizedwith the signals as received, position information data is again derivedfrom the signals from that satellite.

What is needed is an improved spread spectrum receiver, for example, foruse in a GPS system for terrestrial navigation which does not requirethe complexity or costs of conventional systems. In particular, a GPSreceiver is needed which can make optimal use of signals available indifficult environments, such as urban environments, with less than 4satellites continuously in view and frequent obscuration of signals fromsome of the satellites.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides an improvedterrestrial navigation system using a GPS receiver which can continue tonavigate with continuous GPS data from less than the 3 or 4 GPSsatellites commonly required. The GPS data is augmented with data fromanother source. The source of the augmentation data may include datafrom external sensors, data bases including map data bases, and/orknowledge of the physical environment within which the vehicle is to benavigated. The use of such augmentation data permits GPS satellitenavigation solutions for stand-alone GPS systems as well as for GPSsystems integrated with external sensors and/or map databases with lessthan 3 or 4 continuously visible GPS satellites.

In another aspect, the present invention provides a GPS receiver inwhich map data used to determine routing is also used as a source ofdata augmentation for a single satellite solution by providing directionof travel information.

In still another aspect, the present invention provides a method ofaugmenting GPS data using information from the physical environment. Forexample, vehicles are usually constrained to tracks no wider than thewidth of the roadway--and often to tracks only half the width of theroadway--and trains are constrained to the width of their tracks. Thiscross track constraint data may be used to provide augmentation data andallow the vehicle to continue to navigate with only a single satellitein view. The cross track constraint data permits the computation ofalong track data useful for calculating total distance traveled toprovide a GPS based odometer measurement.

The present invention permits the computation of distance along trackfor use as an odometer reading while tracking only one satellite. Crosstrack hold provides along-track data directly which, in the case of avehicle, directly provides distance traveled information useful in lieuof a conventional odometer reading.

In addition to clock and altitude hold, the present invention uses atechnique which may be called cross-track hold in which the singlesatellite in view is used for determining the progress of a vehicle suchas a car along its predicted track, such as a roadway. The dataconventionally required from a second satellite is orthogonal to thetrack and therefor represents the appropriate width of the roadway. Thisvalue may be assumed and or constrained to a sufficiently small value topermit an estimate of the value, e.g. y_(est) to provide a modedescribed herein as cross-track hold while obtaining useful GPSnavigation from a single satellite in view.

In other words, in accordance with the present invention, singlesatellite navigation may be achieved by using the data from the singlesatellite for on-track navigation information while holding orestimating the time, altitude and/or cross-track navigation data.

The required augmentation data may additionally, or alternatively, bederived from other sources in the physical environment, such as turnsmade by the vehicle during on-track travel. In accordance with anotheraspect of the present invention, the vehicle may detect turns madeduring travel and update the current position of the vehicle at the turnin accordance with the timing of the turn. Turn detection may beaccomplished by monitoring changes in the vehicle vector velocityderived from changes in the GPS derived position information or bymonitoring changes in the compass heading or by any other convenientmeans.

In another aspect, the present invention provides a GPS system fornavigating a vehicle along a track, including means for tracking atleast one GPS satellite to provide on-track information related toprogress of the vehicle along a selected track, means for providing anestimate of cross track information related to motion of the vehicleperpendicular to the track, and means for providing vehicle navigationdata, such as vehicle position or vehicle velocity, from the on-trackinformation and the cross-track estimate.

In still another aspect, the present invention provides a method ofderiving position information from a single GPS satellite by tracking atleast one GPS satellite to provide on-track information related toprogress of the vehicle along a selected track, providing an estimate ofcross track information related to motion of the vehicle perpendicularto the track, and determining the position of the vehicle from theon-track and the cross-track estimates.

In still another aspect, the present invention provides a method ofupdating GPS position information for a vehicle navigating on roadwaysby deriving an indication that the vehicle has made a turn at aparticular point along a predetermined track, comparing the turnindication with stored navigation data to select data related to one ormore predicted turns at or near the particular point, comparing the turnindication with the predicted turn data to verify that the indicatedturn corresponds to the predicted turn, and updating GPS positioninformation to indicate that the vehicle was at the predicted turnlocation at a time corresponding to the turn indication.

In still another aspect, the present invention provides a GPS system fornavigating a vehicle, the system including means for tracking at leastone GPS satellite to provide on-track information related to thedirection of travel of the vehicle along a selected track, and means forderiving vehicle navigation data from changes in the direction of travelof the vehicle along the selected track.

In a still further aspect, the present invention takes advantage of thetypical improvement in satellite visibility possible in urban roadwayintersections by providing a fast satellite reacquisition scheme whichpermits data from otherwise obscured satellites to aid in the navigationsolution even though visible only for a short time, for example, as thevehicle crosses an intersection in an urban environment in which tallbuildings obscure the satellites from view except in the intersection.

In a further aspect, the present invention provides a spread spectrumreceiver having means for providing a plurality of versions of a locallygenerated signal related to a spread spectrum signal to be received,means for combining at least two of the versions of the locallygenerated signal with the spread spectrum signal to produce a productsignal related to each of the at least two versions, means forevaluating the at least two product signals to adjust a parameter of thethird version of the local signal, means for combining the adjustedthird version of the local signal with the spread spectrum signal toproduce a data signal, means for determining a predicted value of theparameter when the spread spectrum signal becomes unavailable, means forcombining an additional plurality of versions of the locally generatedsignal related to the predicted value with received signals to produceadditional product signals related to each of the additional pluralityof versions of the locally generated signal, means for evaluating theadditional product signals to produce a reacquired data signal.

In another aspect, the present invention provides a method of operatinga receiver for coded GPS signals from satellites by correlating early,prompt and late versions of a locally generated model of the code withsignals received from GPS satellites to adjust a delay of the promptversion to track a selected satellite, maintaining a predicted value ofthe delay when the selected satellite is unavailable, correlating aplurality of different early versions of the locally generated code withsignals received from satellites to produce correlation products,correlating a plurality of different late versions of the locallygenerated code with signals received from satellites to producecorrelation products, and reacquiring the previous unavailable selectedsatellite by selecting the version producing the largest correlationproduct above a predetermined threshold as a new prompt version of thecode to track the satellite.

In a still further aspect, the present invention provides a spreadspectrum receiver for a spectrum spreading code having a fixed number ofbits repeated during a fixed length time period from a plurality oftransmitters having a first time slicing level for slicing the timeperiod of the transmitted code into a number of time segments evenlydivisible into the twice the number of samples, a second multiplexinglevel for dividing each time segment into a number of channels, each ofthe channels being used for tracking one of the transmitters, and athird level dividing each of the channels in one of the segments into anumber of code phase delay tests.

In another aspect, the present invention provides a receiver forprocessing signals from a plurality of sources, each modulated by adifferent spectrum spreading code repeating at a common fixed interval,including a sampler for deriving digitally filtered I and Q samples froma composite of spread spectrum signals received from the plurality ofsources, means for segregating samples of the signals being receivedduring each interval into a number of time segments, a time divisionmultiplexer for segregating different versions of the sequential samplesinto each of a number of channels, each channel representing one of theplurality of sources, a correlator for correlating the version of thesample in each channel with a series of sequentially delayed versions ofthe spectrum spreading code applied to the signals from the sourcerepresented by that channel, and an accumulator associated with each ofthe series of delays in each of the channels for processing the resultsof correlations performed during one or more intervals to deriveinformation related to the signals.

These and other features and advantages of this invention will becomefurther apparent from the detailed description and accompanying figuresthat follow. In the figures and description, numerals indicate thevarious features of the invention, like numerals referring to likefeatures throughout both the drawings and the description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overview illustration of the operation of a car navigationsystem according to the present invention.

FIG. 2 is a block diagram of the GPS car navigation system depicted inFIG. 1, used for improved navigation during reduced satellitevisibility.

FIG. 3 is a schematic representation of a single satellite channel of aGPS receiver used for fast satellite reacquisition.

FIG. 4 is a schematic representation of a portion of the singlesatellite channel shown in FIG. 3 in which an additional plurality ofsets of delayed code samples are correlated to provide a finer gradationof correlation intervals.

FIG. 5 is a functional block diagram of a preferred implementation, onan ASIC, of the satellite tracking channels and associated processingcomponents of the GPS car navigation system shown in FIG. 1.

FIG. 6 is a functional block diagram of the Doppler Block of the GPS carnavigation system shown in FIG. 1.

FIG. 7 is a functional block diagram of the Coder Block of the GPS carnavigation system shown in FIG. 1.

FIG. 8 is a functional block diagram of the Correlator Block of the GPScar navigation system shown in FIG. 1.

FIG. 9 is a function block diagram overview showing the interconnectionsbetween the Doppler, Code, Correlator and other blocks of the systemdescribed in FIG. 5.

FIG. 10 is a block diagram of the operation of the system, shown inFIGS. 5 and 9, illustrating the data path of the present invention.

FIG. 11 is a series of exploded time segments illustrating the operationof the data path of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1 is an overview illustration of the operation of a GPS carnavigation system according to the present invention. The GPS carnavigation system, described below in greater detail with respect toFIG. 2, is mounted in car 10 which is moving along the center of roadway12. NAVSTAR satellite 14, in the lower left quadrant of the figure, isin view of car 10. A simulated GPS circular overhead display, positionedapproximately over intersection 22 of roadway 12 and roadway 16indicates that satellite 14 is between 0° and 45° degrees of elevationabove the horizon as viewed from car 10.

For the purposes of illustration, satellite 18 is positioned overheadbetween the elevation angles of 0° and 45° degrees. However, the line ofsight between satellite 18 and car 10 is obscured by buildings 20 sothat satellite 18 is not in view of car 10 at the position along roadway12 as shown. Similarly, the line of sight between satellite 19 and car10 is obscured by buildings 21. However, as will be discussed below,when car 10 crosses intersection 22, the line of sight between satellite19 and car 10, when the car is in position 11 within intersection 22,may momentarily be clear.

Turning now to FIG. 2, GPS car navigation system 24 is a firstembodiment of a car navigation system according to the present inventionwhich may be installed in car 10 of FIG. 1. GPS car navigation system 24includes GPS car system module 26 which is provided with signalsreceived from satellites by GPS antenna 28, data related to the thencurrent--and expected future--physical environment of car 10 by forexample map data base 30 and data input from the operator of the car byfor example input device 32. GPS car system module 26 provides output tothe operator, for example, in the form of a GPS map display, via displayunit 34, which may include both visual display as well as a voiceinterface announcing information as required to supplement or evenpartially replace visually presented data.

The present invention may be configured for use with only a GPSreceiver, a GPS receiver aided by map data from, for example, map database 30, and/or a GPS receiver aided by both a map data base as well asan external source of information, for example, from an external sensor.This external source of information may be used for maintainingposition-information by dead reckoning during those times when asufficient number of satellites are not in view to provide the desiredinformation.

In operation, a composite of all signals received from NAVSTARsatellites is applied by GPS antenna 28 to satellite receiver section 36of GPS car system module 26. Signals from individual NAVSTAR satellitesare then tracked in satellite specific tracking channels such as SatTRAKchannels 38, 40, 42 and 44. Although it is quite conventional to track 4to 12 satellites and therefore use 1 to 12 satellite tracking channels,only 4 such channels are shown herein for clarity. The outputs of thesesatellite specific tracking channels are processed by SatProcessor 46 toprovide x_(user), y_(user), z_(user) and t_(user) data via appropriatelogic control to a GPS position processor, such as PosProcessor or NavSoln 48 which determines the navigation solution to determine positiondata. Position data is then applied by PosProcessor 48 to an appropriatedisplay for the operator of the car, such as display unit 34.

External sensor 49, in FIG. 2, may conveniently provide sensor data, orlocal or satellite position information, or position information whichprovided local position or satellite position information directly toPosProcessor 48 for comparison with the position information determinedby SatProcessor 46 and/or Map/Display Processor 50. External sensor 49may conveniently be any sensor which provides information useful forupdating position information for dead reckoning including direction,speed, velocity or acceleration or other data from which dead reckoningdata may be derived. Conventional sensors include inertial navigationsystems, with magnetic or optical gyroscopes, fluxgate compasses,odometer or wheel sensors or the like. Alternatively, external GPSformat signals, such as those provided by a pseudolite, may be used toupdate current satellite or position information.

At the beginning of a navigated trip, the operator of car 10 wouldtypically provide data concerning the physical environment surroundingthe intended route to GPS car system module 26 by inserting anappropriate data storage device such as a CD ROM, into map data base 30,and/or by entering the data via input device 32 which conveniently maybe a keypad, keyboard, pointing device, track ball, touch screen,graphical pad, a voice recognition interface and/or a combination ofsuch input devices. The operator of car 10 would also enter the intendeddestination into GPS car system module 26 via a data entry device suchas a mouse or track ball interacting with display unit 34 and/or viainput device 32. Map/Display Processor 50 of GPS car system module 26would then develop the desired route, typically from the then currentlocation as a point of origin to the desired destination, in accordancewith the rules of navigation and details of the locale provided by mapdata base 30. The appropriate route data is stored in Route Data Base52, including the routing in the form of roadways and turns betweenroadways. Additional information, such as altitude, width of theroadways and etc. may also be contained within map data base 30 and/orRoute Data Base 52. These data bases may be contained within GPS carnavigation system 24 and/or made available to GPS car system module 26from outside storage media such as diskettes positioned in appropriatedisk drives.

During navigation, each satellite in view may be tracked in a satellitetracking channel. If, for example, 4 or more satellites are in view,each of the satellites in view will be tracked in an individual channel,such as SatTRAK channels 38, 40, 42 and 44. The output of the satellitetracking channels is then applied to SatProcessor 46 which would providesatellite based solutions of the four unknowns, such as x_(user),y_(user), z_(user) and t_(user). The data represented by x_(user) andy_(user) are conventionally used as the two dimensional orthogonalcomponents of the surface of the earth such as north and east. However,in accordance with the present invention, x_(user) and y_(user) arepreferably used to represent data for a pair of orthogonal directionsspecific to the direction of vehicle travel called the on-track andcross-track directions.

Conventional bearing, such as north, south, east and west are relativeto the magnetic or true north poles of the earth, while on-track andcross-track, as used in the present invention, are bearings maderelative to the expected direction of travel of car 10 at any particularpoint in the route. For example, while a 90° turn from a heading of duenorth would change the angle of the vehicle velocity vector from 0° to90° if bearings relative to the Earth's surface such as north and eastare used, the same turn would show no change in the 0° angle of thevehicle velocity vector before or after the turn as long as car 10remained on the expected track.

The data represented by z_(user) is typically surface elevation, such asthe elevation above sea level, while the data represented by t_(user) isthe exact time as determined from one or more of the satellite trackingchannels.

Solutions for all 4 unknowns of position information may be derived fromsignals from 4 satellites in view, so that exact position informationwithin the limit of the accuracy then available from the GPS satelliteconstellation in view can therefore be applied by PosProcessor 48 toMap/Display Processor 50. The position information determined from thesatellites is processed with the physical data from map data base 30,and/or the desired routing data from Route Data Base 52, to provideappropriate navigation information to the operator of car 10 via displayunit 34.

If less than 4 satellites are in view, the t_(user) solution applied toPosProcessor 48 may be replaced by t_(est) 54 estimated solution derivedfor example from an internal clock model 54 in position estimate ormodel 63. Similarly, the Z_(user) solution may be replaced by z_(est) 56solution derived from elevation estimate 56, also in position model 63,in accordance with routing data derived from Route Data Base 52 inaccordance with then current GPS position information applied toMap/Display Processor 50. T_(est) 54 and z_(est) 56 are applied toPosProcessor 48, and used in lieu of t_(user) and z_(user), when onlytwo satellites are in view. The use of estimated or modeled solutionsfor the t and z variables, that is the use of t_(est) 54 and z_(est) 56are conventionally known as clock hold and altitude hold, respectively.

It must be noted that the particular configuration of GPS car navigationsystem 24 as described so far is only one of the many known ways ofconfiguring such systems any of which may be used without departing fromthe spirit or scope of the present invention as defined by the claims.

In accordance with the present invention, the width of the roadway,either known or estimated, may be used to provide y_(est) 60 for use inlieu of y_(user) when only one satellite is visible. y_(est) 60 may bederived from Route Data Base 52 and/or map data base 30. Since the x andy unknowns are orthogonal, x_(user) may be used to describe the on-trackinformation, that is, the progress of car 10 along its predeterminedtrack while y_(est) 60 represents the cross track information, that is,how far car 10 has strayed from the center of the roadway.

Referring therefore to FIG. 1, x_(user) is used to indicate the progressof car 10 along roadway 12 while y_(est) 60 is used to represent thewidth of roadway 12. The actual width of the roadway may be derived frommap data base 30, or assumed because the actual value of the width ofthe roadway is relatively small and often therefore insignificantcompared to the distances to be measured along the navigation route.Since the maximum allowable cross-track error, i.e. the maximumallowable appropriate value for y, is constrained by the physical widthof the roadway, y_(est) 60 is relatively easy to accurately estimate.

By using y_(est) 60, z_(est) 56 and t_(est) 54, it is possible toprovide useful navigation data for car 10 along a known roadway usingsignals from only a single satellite in view. It is important to notethat reasonably accurate prior or initial position information may berequired and that not all visible NAVSTAR satellites will be suitablefor single satellite navigation, depending upon the position of thesatellite with respect to the path of car 10. The position informationdetermined during single satellite navigation is along track positioninformation which may be accumulated and used for determiningaccumulated along track distance traveled. This data provides, and maybe used in place of, the distance traveled information conventionallyprovided in a vehicle by an odometer.

Referring now to both FIGS. 1 and 2, turning data may be used to improveterrestrial GPS navigation by using the detection of a known turn toupdate progress along a predetermined route. When at least 4 satellitesare in view, the position of car 10 may be known to the accuracy of theGPS system. When using clock, altitude or cross-track hold, or somecombination thereof, the known position of the car is degraded by anyinaccuracy of the estimate or estimates used. For example, during clockhold, internal clock model 54 drift and inaccuracy of the source oft_(est) 54 will degrade the accuracy to which the position is known as afunction of the magnitude of the inaccuracy. Similarly, any change inaltitude from the estimated or fixed altitude, that is, any inaccuracyof z_(est) 56, will degrade the accuracy of the known position. Changesin roadway width and inaccuracies in the map data with regard to theroadway width, that is, any inaccuracy in y_(est) 60, may also degradethe position information.

Even with 4 satellites in view, the geometry of the visible satellitesmay make it difficult to determine position by measurement of GPSsignals. Further, during terrestrial navigation, it is not uncommon forsatellites to be temporarily obscured from view during navigation by,for example, being blocked by buildings and other obstructions.

It may therefor be desirable to update the accuracy with which thecurrent position of the vehicle is known with actual positioninformation whenever possible. The update information will sometimes beuseful when 4 satellites are in view, but will always be useful assupplemental data when less than 4 satellites are in view. Updateinformation is extremely useful during single satellite navigation toavoid the accumulation of errors in position information.

In operation, an original position and a destination were provided tothe system which then determined the track to be followed. The track tobe followed, or routing information, may be provided in the form of adata base of route information such as Route Data Base 52. In theexample used, track 62 follows the centerline of roadway 12 tointersection 22 and then makes turn 64 to follow the centerline ofroadway 16. Track 62, roadways 12 and 16, intersection 22 and turn 64are provided to Route Data Base 52 during the preparation of the routeby Map/Display Processor 50 from the then current position and thedestination entered via input device 32.

The physical position of car 10 is very accurately known when car 10turns at turn 64. This accurate knowledge of the car's position at aparticular time may conveniently be used to update the GPS navigationinformation by providing a position reset which is similar to a knowninitial position. Update information from turns will most likely beuseful if the angle of turn 64 is sufficiently large to provide anunambiguous position determination. It is expected that any turn greaterthan 45° may be detected. As the speed of the vehicle increases, smallerturn angles may also provide useful information. The position updateinformation is applied to position model 63 to update internal clock ort_(est) model 54, elevation or z_(est) model 56, y_(est) model 60 aswell as x_(est) 61 which is a model of the along-track position of thecar. These four estimates together for position model 63, which may beupdated by information from map data base 30, Route Data Base 52,current position processor 70, PosProcessor 48 and/or external sensor49, to form the most accurate available position model 63. Positionmodel 63 may also be used to provide estimates to the same data sources.

The actual turning of the car may be detected by a change in the vehiclevelocity vector determined from the GPS data or from other conventionalmeans such as a magnetic compass or an inertial navigation sensor. Inaccordance with the rapid reacquisition system described below withrespect to FIG. 3, GPS data alone may conveniently detect such turnseven when single satellite navigation is required. The turn as detectedby turn detector 66 is correlated with data from Route Data Base 52 todetermine the actual position of the car to the accuracy of the map database 30. The accuracy of the data in map data base 30 may easily andconveniently be much greater than the accuracy available from the GPSsystem especially if single satellite navigation, or any combination ofclock, altitude or cross-track hold, is used. Therefore the positionupdate may provide a substantial increase in the accuracy of the thencurrent position determination.

The benefit of the approach of this embodiment of the present inventionis similar to the identification and use of a known waypoint during adead reckoning navigation run. The cumulative error is reducedsubstantially at the known waypoint so that additional, future positiondetermination errors do not carry the burden of an accumulation of pasterrors.

As shown in FIG. 2, Route Data Base 52 provides data related to track62, typically from Map Data Base 30, to Map/Display Processor 50 todisplay the current GPS position and may also provide similarinformation to turn detector 66, turn comparator 68 and/or currentposition processor 70 in order to update PosProcessor 48 with a positionreset.

Turn detector 66 may be configured in many different ways and is used todetect turns actually made by car 10 and select turns, such as turn 64,from Route Data Base 52 for later comparison with the detected turn. Inaccordance with a preferred embodiment of the present invention, turndetector 66 may operate on the current GPS position provided byPosProcessor 48 to develop a vehicle velocity vector position indicatingboth the direction and speed of travel. Substantial changes in thedirection portion of the vehicle velocity vector would indicate a changein direction, such as a turn. Turn detector 66 may therefore detectturns directly from the GPS information by determining the vehiclevelocity vector and detecting changes in the vehicle velocity vectorwhich represent a turn.

Turn detector 66, or another unit if convenient, also operates on theroute information provided by Route Data Base 52 to determine theexpected position of car 10 along track 62 based on the then current GPSposition information. Once the expected location of car 10 along theroute is determined, one or more turns in the area of the expectedposition of car 10 can be selected for comparison with the indicationsof a physical turn derived from the GPS data.

When changes in the actual vehicle velocity vector, as derived forexample from the GPS position data, compare appropriately with thechanges predicted at a particular turn as derived from Route Data Base52, the actual position of car 10 at the time of the turn can be veryaccurately determined and used to update the GPS data at the turn. Forexample, if an actual turn is detected from a change in the vehiclevelocity vector from the GPS position of car 10 near the time predictedfor that turn, the actual position of car 10 at the time of the turn canbe determined and used to update the then current GPS position for useas a position reset applied to PosProcessor 48.

Alternatively, turn detector 66 may use non GPS measurements fordetermining the occurrence of an actual turn of car 10, such as compassheadings or inertial navigation determinations derived for example fromexternal sensor 49, and applied directly to turn detector 66 or viaPosProcessor 48 as shown in FIG. 2.

Detection of turns from GPS signals may easily be accomplished as longas 2 satellites are in view and provide appropriate geometries fordetermining two dimensional coordinates of the car's position. Duringsingle satellite navigation, as described above, the use of turninformation for updating the last known position information becomeseven more important, but the location of the single satellite in view,relative to track 62, becomes of even greater importance so that actualturns may be accurately detected.

Turn detection may also be provided by monitoring changes betweenacquired and obscured satellites. If, for example, only satellite 14 wasvisible to car 10 on roadway 12 before intersection 22, and uponentering intersection 22, satellite 19 suddenly became visible whilesatellite 14 was momentarily obscured, the change over from satellite 14to satellite 19 could be used to indicate a turn in accordance with thedata from each satellite. Using a rapid reacquisition scheme, asdescribed herein below, the actual position at which the change ofdirection, that is, the switch between satellites, can be sufficientlyaccurately determined to permit precise position update information atthe turn.

Similarly, turn comparator 68 may conveniently be implemented withinanother component of the system, such as PosProcessor 48, Map/DisplayProcessor 50 and/or SatProcessor 46, so that a candidate turn may beselected from the route data for track 62 for comparison with thedetected turn data.

Referring now to FIG. 3, in another embodiment, the present inventionprovides for fast reacquisition of satellite signals, useful for examplewhen a previously acquired satellite is obscured and then appearsperhaps for only a short time, for example, as a car travels through anintersection.

Referring to the line of sight between car 10 and satellite 19 as shownin FIG. 1, it is common in an urban environment for the buildings alongthe sides of the street to act as a barrier wall obscuring the lines ofsight to many GPS satellites. However, the barrier wall formed bybuildings 20 and 21 is commonly breached at intersections such asintersection 22. For example, car 10 while traversing intersection 22may reach position 11 in which the previously obscured line of sight toa satellite, such as satellite 19, is momentarily not obscured becauseof the break between buildings 20 and 21 at intersection 22. Thismomentary visibility of a previously obscured satellite may occur whilecar 10 is in the intersection or at the edges of the intersection.

The length of the momentary contact with satellite 19 is relativelyshort. For example, if intersection 22 is 60 feet wide and car 10 istraveling at 30 mph, the time taken to cross the intersection may be asshort as 1.3 seconds. Conventional GPS navigation systems would notreacquire and derive useful data from satellite 19, even if previouslyacquired, during this short time interval.

In accordance with another embodiment, the present invention makesmaximum use of such reacquisition opportunities by minimizing the timerequired for reacquisition, the collection of data and processing of thecollected data for position determination. Referring now to FIG. 3, aportion of SatTRAK channel 38 is shown in greater detail as an exampleof the configuration of each of the satellite tracking channels. Afteroriginal acquisition, SatTRAK channel 38 tracks a single satellite byoperating on satellite signals 72 received by GPS antenna 28. Satellitesignals 72 include the signals from the satellite being tracked bySatTRAK channel 38 and are demodulated and selected by being multiplied,in one of the correlators 74, by a copy of the 1023 chip pseudorandom,spread spectrum code applied to satellite signals 72 by the GPSsatellite. Correlators 74 may be configured from exclusive OR or NORgates to minimize the time required for providing a correlation result.

During tracking, the copy of the code produced by code generator 76 andapplied to exclusive OR correlators 74 by delay 78 is synchronized withthe code in satellite signals 72, as received, so that the copy of thecode correlates with satellite signals 72. This may be accomplished inseveral different manners known in the art, including by shifting thetime of generation of the code in code generator 76 and/or adjusting theamount of delay applied by an external delay. In any event, the codeapplied to exclusive OR correlators 74 when SatTRAK channel 38 is lockedto the selected satellite, is synchronized with the code being receivedfrom that selected satellite. This correlation is commonly called theon-time or prompt correlation to indicate this synchronization.

Conventional GPS receivers maintain a lock on a satellite signal afteracquisition by performing additional correlations, often called earlyand late correlations or correlations performed by early and latecorrelators. These correlations are displaced in time by a certain delaysuch as one half the width of a C/A code chip from the on-time or promptcorrelator. That is, if the time of occurrence of a particular chip inthe satellite signals is time t0, the prompt correlator under idealconditions would multiply satellite signals 72 with a replica of thecode with the same chip at time t0. The early correlation would beperformed at time t0-1/2 chip and the late correlation would beperformed at a time equal to t0+1/2 chip. Whenever the synchronizationbetween code generator 76 and satellite signals 72 as received begins todrift, the correlation results begin to change in favor of either theearly or late correlation at the expense of the prompt correlation.

One conventional approach to maintaining lock on the signals from aparticular satellite is to adjust the timing of code generator 76 with afeed back loop used to maintain the power in the correlation products inthe early and late correlators to be equal. In this way, code generator76 may be continuously resynchronized with satellite signals 72 so thatthe accuracy of the system is within one half chip in either direction(early or late) of the signals received.

When satellite signals 72 are temporarily lost, for example, because thesatellite signals are temporarily obscured by buildings 20 and 21 asshown in FIG. 1, various techniques are used to attempt to synchronizecode generator 76 with satellite signals 72 as received so that SatTRAKchannel 38 can reacquire the signals from the desired satellite. Asnoted above, conventional techniques include clock and altitude hold andone embodiment of the present invention provides another techniquecalled cross-track hold.

However, unless the obscuration of the satellite signals is very brief,the accuracy of prediction of such techniques is not enough to maintainsynchronization exception for a very brief period of obscuration.

In accordance with another embodiment of the present invention,massively parallel correlation is used to create an expanded capturewindow of correlation capture around the then current predictedsynchronization time in order to immediately reacquire a previouslyacquired, and then obscured, satellite signal. In particular, the speedof reacquisition is made sufficiently fast according to the presentinvention so that useful GPS position data may be acquired during thetime car 10 travels through intersection 22 even though, for example,the signals from satellite 19 were obscured by buildings 20 until car 10was within intersection 22.

To this end, an expanded series of correlations are performed with aseries of delays a fixed fraction of a chip width, such as 1/2 chipwidth, apart extending both early and late of the predicted promptcorrelation. As shown in FIG. 3, satellite signals 72 are devolved intoa fixed number of samples, by for example analog to digital conversionin A/D Converter 73, to provide n Signal Samples 75. A similar number ofcode samples are provided through k fixed 1/2 chip width delays 78 toprovide k-1 sets of n Code Samples 80, progressing from a first set of nCode Samples 80 with no delay to the k-1st set of n Code Samples 80which have been delayed by a total of k delays 78. It is convenient touse 1/2 chip delays for each delay 78, but other fractions of a chipwidth may be used.

The k/2th set of n Code Samples 80, or the set nearest k/2, mayconveniently be delayed the correct amount to perform the promptcorrelation in one of the exclusive OR correlators 74 with n SignalSamples 75 from A/D Converter 73 during tracking. The k/2th-1 set of nCode Samples 80 may then be used to perform the early correlation whilethe k/2th+1 set of n Code Samples 80 may then be used to perform thelate correlation while tracking. The additional correlations may also beperformed during tracking, but provide a substantial advantage when usedduring reacquisition.

That is, in the present invention, the early, prompt and latecorrelations conventionally used in tracking may also be used duringreacquisition mode, aided by a substantial number of correlations usingadditional delays. Whether or not the early and late correlations areused, a convenient number of additional delays on each side of theprompt delay results from (k-1)=20 so that nine or ten 1/2 chip delaysare provided on each side of the k/2th prompt delay. In this way,correlations are performed during reacquisition at time delays of 5 chipwidths on either side of the predicted prompt or on-time delay. Thisrepresents an expanded capture window of on the order of ±5×300 metersof potential error. That is, if the predicted synchronization withsatellite signals 72 modeled by GPS car system module 26 drifted by asmuch as the equivalent of a ±1500 meter position error during signalloss from a particular satellite resulting from, for example obscurationin an urban setting, at least one of the plurality of exclusive ORcorrelators 74 would provide the required prompt correlation toimmediately lock onto satellite signals 72.

Once the correlations are performed, the correlation results for eachset of n Code Samples 80 are summed in summers 84 to produce a series ofvalues each separately indicating the correlation of n Signal Samples 75with each of the sets of n Code Samples 80. These correlation resultsare applied to threshold test 82, the output of which is applied toSatProcessor 46 only when satellite signals 72 have been successfullyreceived. The output of threshold test 82 specifies the number of delayswhich represent the prompt correlation for the reacquired satellitesignal. It is important to note that in accordance with the presentinvention, the satellite tracking and reacquisition modes are notseparated functions but rather interact seamlessly. That is, byproviding a substantially expanded capture window, the correlations usedfor tracking are also automatically useful for immediate reacquisitionas long the capture window is sufficiently wide to include any positionerror accumulated during signal obscuration or other loss.

Because the speed of reacquisition is very important in order tomaximize the opportunity to utilize the brief time during travel throughintersection 22 when satellite 19 may temporarily be in view, it isadvantageous to perform all such correlations in parallel. Further, itis advantageous to continuously perform all such correlations in thecapture window in order to minimize time when a satellite signal is notbeing tracked. In accordance with the presently preferred embodiment,exclusive OR correlators 74 are implemented in hardware rather thansoftware to maximize the speed of correlation and minimize any erroraccumulation by minimizing the time for reacquisition.

In operation, when car 10 follows track 62 along roadway 12, during atleast part of the time buildings 21 obscure the line of sight betweencar 10 and satellite 19. If satellite 19 had previously been acquired byGPS car system module 26, an approximate time value to synchronize withthe satellite signals will be predicted. This value is maintained asaccurately as possible within GPS car system module 26 while satellite19 is obscured. In order to maintain the prediction for the requireddelay as accurately as possible, that is, to minimize the position erroraccumulated during signal loss, the above described techniques formaintaining or updating position accuracy by using cross-track hold,resetting position at a determined turn and/or the use of externalsensors for dead reckoning provide a substantial benefit for use withthe combined, expanded tracking and reacquisition windows describedabove.

Present technology makes it convenient to provide 1/2 chip delaysbetween correlators, but other delay values may be used. Similarly, itis convenient to expect that the prompt correlation can be maintainedwithin plus or minus 5 chips of the timing of the satellite signals.FIG. 3 therefore portrays a series of 9 or 10 early and 9 or 10 latecorrelators surrounding prompt correlator 74 to achieve the ±5 chipcapture window surrounding prompt correlator 74 in 20 half chip steps. Adifferent number of correlators and other delays would also work withthe present invention.

Use of a plurality of fixed delays of one half chip width permit theimmediate reacquisition of signals from a satellite to within anaccuracy of one half chip width. In accordance with satellite signals 72as presently provided by the NAVSTAR satellites, one half chip widthrepresents about 150 meters of maximum position error. It is possible tosubstantially reduce the maximum position error, and/or the speed ofprocessing the data, by using fixed delays of a different amount ofdelay, e.g. fixed delays of one third, one quarter, one fifth or someother value of a chip width.

Conventional approaches for different modes of operation, switch betweenwide and narrow delays at acquisition +/or reacquisition in order toprovide a compromise between the width of the capture window and thenumber of correlations required for the desired range. In accordancewith the present invention, a new technique is used which permits theconvenient use of fixed, chip width delays to provide a finer gradationof correlation steps. In particular, as shown in FIG. 4, two sets ofhalf width delays are used to provide the equivalent of a set of quarterwidth delays. The number of sets of fixed delays and the offset betweenthem may be selected in accordance with the requirements of theapplication being addressed.

Referring now to FIG. 4, a first plurality of sets of n Code Samples 80are derived directly from code generator 76, delayed from each other by1/2 chip width delays 78 and correlated with n Signal Samples 75 inexclusive OR (or NOR) correlators 74 as provided in FIG. 3. Forconvenience of explanation and drawing, the outputs from this first setof set of n Code Samples 80 are shown applied to summers 84 to indicatethat the correlation products produced in exclusive OR correlators 74from each such set of n Code Samples 80 are applied to threshold test 82via summers 84. All such correlation products are applied, but forclarity only the correlation products having no delay, the predictedprompt or k/2th delay and the kth delay are depicted. The correlationproducts from this first plurality of sets of n Code Samples 80 arespaced apart by 1/2 chip width delay as noted above.

In addition, in accordance with the present invention, additional setsof correlation products at different spacings are available by use ofone or more additional sets of 1/2 chip delays 78 by, for example,tracking the same satellite in two or more channels offset in time fromeach other. It is important to note again that other delays and/oroffsets may also conveniently be used and the delays need not all be thesame.

In particular, a second plurality of sets of n Code Samples 84 arederived from code generator 76 and delayed from each other by 1/2 chipwidth delays 78. However, the delays in the second sets of n CodeSamples 84 are offset from the delays in the first sets of n CodeSamples 80 by a fixed amount, such as a 1/4 chip width delay, byinsertion of 1/4 chip width delay 79 between code generator 76 and thefirst set of n code samples in sets of n Code Samples 84. This resultsin each of the samples in sets of n Code Samples 84 falling halfwaybetween two of the sets of n Code Samples 80. As shown in FIG. 4 onlyk-1 sets of n Code Samples 84 are required with k sets of n Code Samples80.

Each of the sets of n Code Samples 84 are correlated with n SignalSamples 75 in exclusive OR correlators 74 as provided in FIG. 3 toproduce correlation products which are then summed by additional summers84. As noted above, the dashed lines between each of the sets of codesamples and summers 84 are used to indicate that the correlation productbetween that set of code samples and n Signal Samples 75 is applied to aparticular one of summers 84. As can then easily be understood from FIG.4, correlation products separated from each other by 1/4 chip widthdelays, from the 0th delay to kth delay, are produced using sets of 1/2chip width delays and a single 1/4 chip delay (which may represent theoffset delay between two channels) and after individual summation areapplied to threshold test 82 to determine which delay represents thecurrently prompt delay of satellite signals 72 from a satellite beingreacquired by GPS car system module 26.

The second set of 1/2 chip delays may easily be implemented by having asecond channel track the same satellite, offset, however by 1/4 chipwidth delay 79.

In this way, the range of delay within which a satellite signal lock maybe acquired, maintained and/or reacquired may be reduced from ±1/2 chipwidth, to about ±1/4 chip width, which permits faster pull in to lock,i.e. when the tracking has been optimized and range error reduced tominimum.

It is important to note the seamless integration of tracking andreacquisition provided by the present invention in that the samecorrelations are used for tracking and reacquisition and the relatedspeed of capture and lock and simplicity provided thereby. The abilityto rapidly reacquire within a capture window so that one of thecorrelations may immediately be used as a prompt correlation, speeds upall data acquisitions thereafter. It is also convenient to utilize afirst plurality of sets of n Code Samples 80 for tracking and, whensatellite signals 72 are lost, provide additional accuracy inreacquisition by using a second plurality of sets of n Code Samples suchas sets of n Code Samples 84. In particular, the same plurality of setsof n Code Samples 84 may be used for reacquisition of signals 72 fordifferent satellites at different times in order to reduce the totalnumber of components and steps required to produce all the necessarycorrelations and summations.

In operation, GPS car system module 26 continuously attempts to trackand reacquire the signals from satellite 19 in SatTRAK channel 38 whilesatellite 19 is obscured from view. As car 10 passes throughintersection 22, the line of sight to satellite 19 is momentarily notobscured by buildings 21. Whenever any of the correlations performed inSatTRAK channel 38 indicate that the satellite signals are beingreceived with sufficient strength so that the correlation products fromsome of the correlators are above threshold, reacquisition isimmediately accomplished. Reacquisition occurs when the correlatoroutput indicating the largest magnitude is selected as the new promptcorrelator. Conventional techniques for improving the quality of thedata are then employed.

The data from satellite 19 is used to immediately, after a settling timefor lock, update the GPS data and correct the currently known positioninformation derived therefrom. Even if satellite 19 is then againimmediately obscured, the update information derived during travelthrough the intersection by fast reacquisition provides a substantialimprovement in accuracy of the GPS determined position. This will permitGPS car system module 26 to continue accurate navigation even throughotherwise very difficult areas, such as city streets.

Although the use of single satellite navigation data by cross-track holdand the updating a satellite data by detecting turns and/or immediatelyreacquiring satellite signals in intersections have all been describedseparately, they are also very useful in combination. Terrestrialnavigation systems, using GPS receivers in a stand alone mode, aided bymap displays and data bases and/or aided by external sensors such asinertial navigation systems may benefit from the use of combinations ofone or more such modes. In a preferred embodiment of the presentinvention, all three techniques are combined to maximize the ability ofthe car navigation system to provide accurate and useful navigation datawhile traversing a difficult environment such as city streets.

Referring now to FIG. 5, a preferred embodiment of the present inventionis described in which major portions of SatTRAK channels 38, 40, 42 and44 and SatProcessor 46 of the present invention are implemented in anApplication Specific Integrated Circuit or ASIC 102. Many of thefunctions of a conventional satellite processor may still, however, beperformed in software. The particular implementation depicted provides a12 channel GPS acquisition and tracking system with fast reacquisitioncapabilities as described above while substantially reducing the numberof gates required on the ASIC to implement this system.

The signals received by GPS antenna 28 are digitized and form a digitalcomposite of signals received from all satellites in view to producesample data 100 which is at a frequency of 37.33 f₀ where f₀ is the chiprate of the C/A code applied to each GPS satellite. For convenience, thefrequencies described below will be designated in terms of multiples off₀. Each of 12 Space Vehicles (SVs) or satellites are tracked in ASIC102 under the control of Central Processing Unit, or CPU, 101 whichprovides control signals and data to ASIC 102. In particular, CPU 101provides data regarding the predicted Doppler shifts and C/A codeapplied to each SV to Random Access Memory, or RAM, R1 103 associatedwith ASIC 102 which provides the data to RAM R2 105 at designated times.RAM R2 105 provides data to and receives data from ASIC 102, permittingCPU 101 data updating and ASIC 102 processing of old data to operatesimultaneously. RAM R2 105 is used by ASIC 102 primarily to storeintermediate values of signals during processing. Other conventionalportions of a micro-computer including a CPU are not shown butconveniently may include devices operating software implementing thesingle satellite, cross-track hold and other techniques described aboveas well as other functions of SatProcessor 46.

Sample data 100 is applied to C/A code acquisition, tracking andreacquisition block CACAPT 104 in ASIC 102 where it is split intoin-phase and quadrature-phase, or I and Q, signals at baseband by I/Qsplitter 106. After processing by CACAPT 104, the I,Q signals arerotated for Doppler shift in 12 channel Doppler Block 108 whichseparately compensates for the expected Doppler frequency shifts of eachof the 12 SV's which can be tracked.

The Doppler rotated I,Q signals for each SV are then applied toCorrelator Block 110 where each signal sample, which is from one of the12 SVs, is correlated in a multiplexed fashion with 20 delayed versionsof the C/A code, produced by 12 channel Coder Block 112, for that SV.During each segment of time, as described below with regard to FIG. 11in greater detail, Correlator Block 110 performs 240 C/A codecorrelations in accumulator 175 to enhance the speed of acquisition andreacquisition. The output of Correlator Block 110 is applied to IQACCUMBlock 114, and the output of IQACCUM Block 114 is applied to IQSQACCUM116, in Accumulator Block 115. IQACCUM Block 114 is convenientlyconfigured from another block of RAM associated with ASIC 102,identified herein as RAM 3. Similarly, IQSQACCUM 116 is convenientlyconfigured from another block of RAM associated with ASIC 102,identified herein as RAM 4.

Accumulator Block 115 operates in different fashions during acquisition,tracking and reacquisition modes under the direction of CPU 101. Duringacquisition mode, Coder Block 112 is caused to sequence through as manysets of 240 different codes delays as necessary to acquire the satellitesignals from a particular space vehicle. That is, as many sets of 240different delays are correlated in Correlator Block 110 to provideIQSQACCUM 116 with an appropriate correlation output whose powerindicates that correlation has been achieved with that satellite. Theprocess is then repeated for each satellite to be acquired. Forconvenience, all delays may be tested.

During reacquisition, a single set of 20 delays are correlated inCorrelator Block 110 to determine if one such delay provides a peakvalue above a predetermined threshold to indicate that a correlation hasbeen achieved and the satellite thereby reacquired. The reacquisitionmode operates transparently within the tracking mode in that a set of 20delays are correlated in Correlator Block 110. If tracking ismaintained, the peak signal may migrate from a particular delay to thenext adjacent delay but will be maintained within the current set of 20delays being correlated. It is convenient to consider the delayproducing the signal with the greatest magnitude as the promptcorrelation product. The signals produced by one more and one less delaythen become the early and late correlation products which may beprocessed in a conventional manner to maintain lock with each satellite.

If the signal from the satellite is temporarily obscured or lost for anyother reason, the then current set of 20 delays is correlated to searchfor a peak of sufficient magnitude to indicate reacquisition. Thedoppler and code values are continuously updated based upon the lastavailable position information including velocity, and the correlationsare performed, until the satellite signal is reacquired or sufficienttime has elapsed so that the satellite signal is considered lost.

The operation and configuration of ASIC 102 will now be described ingreater detail with regard to the in-phase or I signal path. Thequadrature phase or Q signal path is identical and need not beseparately described.

Within CACAPT 104, sample data 100 is applied at 37.33f₀ to I/Q splitter106 to produce a 2 bit signal at 18.67f₀ which is further reduced to 2f₀by Digital Filter 118 which operates by adding sets of 10, 9 and 9samples which are summed, quantized, and then stored serially in 11sample deep buffer 120. When 11 sample deep buffer 120 is filled, thedata is transferred in a parallel fashion to an identical buffer, calledparallel block 122, for Doppler rotation. Data is therefore transferredout of 11 sample deep buffer 120 when 11 samples are received, that is,at a chip rate of 1/11th of 2f₀ or approximately 0.18f₀. 11 sample deepbuffer 120 operates as a serial to parallel converter while parallelblock 122 operates as a parallel to serial converter. This results in186 parallel transfers per msec.

Data is shifted out of parallel block 122 at 24f₀ to 12 channel DopplerBlock 108 so that the Least Significant Bit or LSB of the serialconverter, parallel block 122, is the output of CACAPT 104 in the formof CapIOut and CapQOut which are applied as CACAPT Data output 123 to 12channel Doppler Block 108. The increase in chip rate from 2f₀ to 24f₀provides an operating speed magnification of 12 as will be describedbelow in greater detail.

Referring now also to FIG. 6, 12 channel Doppler Block 108 is nowdescribed in greater detail. Doppler Block 108 receives satellitespecific CACAPT Data output 123 including CapIOut and CapQOut fromCACAPT 104 for storage in Doppler Register 124. Satellite or sourcespecific predicted Doppler phase, after processing by Carrier NumericalControl Oscillator or NCO 125 and sine/cosine look-up table 134, is alsoapplied to Doppler Register 124 where it is added to CapIOut and CapQOutfor the same SV (or other source) to form dopIOut and dopQOut. WithinDoppler Block 108, Carrier₋₋ NCO 125 operates at an effective rate of2f₀ for each satellite channel because the data sample rate is 2f₀.

For each SV, CPU 101 stores the satellite specific predicted carrierphase dopPhaseParam, and predicted carrier frequency dopFreqParam, inRAM R2 105. Sat₋₋ Mem 186 then transfers the dopPhaseParam anddopFreqParam as shown in FIG. 9 to Carrier Phase Register 126 andCarrier Phase Output Buffer 128, respectively, at each 1 msec boundary.In the drawings, the number of the first and last bit of the signal isprovided in parenthesis, separated by a full colon, in accordance withcurrent conventions. Therefore, dopFreqParam is a 24 bit digital value,the MSB of which is bit number 23 and the LSB of which is bit number 0.Adder 130 adds carrier phase to carrier frequency, derived fromdopPhaseParam and dopFreqParam, to produce the current carrier phasevalue in Carrier Phase Register 126 shown as Carrier₋₋ NCO.

The four Most Significant Bits or MSB's of Carrier₋₋ NCO in CarrierPhase Register 126 are applied to sine/cosine look-up table 134 whichincludes 2 4-bit registers for storing its output. The output ofsine/cosine look-up table 134 is applied to Doppler Multiplier 132 inDoppler Register 124 for Doppler rotation of CACAPT Data output 123(CapIOut and CapQOut) to produce rotated SV output signals dopIOut anddopQOut. Doppler Register 124 uses Doppler Multiplier 132, as well asfour 4-bit registers, two adders, another pair of 5-bit registers and aquantizer to form dopIOut and dopQOut. Referring for a moment to FIG.11, dopIOut and dopQOut are applied to parallel converter 166 androtated SV output signal 127 is the output of serial to parallelconverter 166 which is applied directly to 11 bit Holding Register 140.

During each segment of time, the beginning value for the Doppler phaseof each SV is stored in RAM R2 105, retrieved therefrom by Doppler Block108 for the rotation of the SV during that segment. At the end of eachsegment, the end value of Doppler phase is stored in RAM R2 105 for useas the beginning value for the next segment. Under the control of gpsctl182, Doppler phase value dopP₋₋ Next in Carrier Phase Output Buffer 128,saved at the end of each rotation for a particular SV by dopSave, isapplied to Sat₋₋ Mem 186 for storage in RAM R2 105 for that SV, to beretrieved by Doppler Block 108 again during the next Doppler rotation ofthat SV in the following segment. The operation of Multiplexer Block 129may be best understood from the description of the triple multiplexingof ASIC 102 associated with FIGS. 10 and 11.

Referring now also to FIG. 7, 12 channel Coder Block 112 includesCoder₋₋ NCO 136 and Code Generator 138. Coder₋₋ NCO 136, which issimilar to Carrier₋₋ NCO 125 shown in FIG. 6, creates Gen₋₋ Enablewhenever Phase Accumulator 148 overflows. Gen₋₋ Enable is the MSB of theoutput of Phase Accumulator 148 and is applied to Code Generator 138.

In particular, under the control of gpsCtl 182, Sat₋₋ Mem 186 appliesthe satellite specific 24 bit code frequency parameter, coderFreqParam,and the 24 bit satellite specific code phase parameter, codePhaseParam,at each 1 msec edge to Coder₋₋ NCO 136 from RAM R2 105. CoderFreqParamis added to codePhaseParam effectively at 4f₀ per channel in Phase Adder150 even though codePhaseParam operates at 48f₀ during tracking andreacquisition. A pulse can be generated for Gen₋₋ Enable between 0 Hzand 4f₀ Hz. In order to generate Gen₋₋ Enable at 2f₀, the value of halfthe bits (23:0) of Phase Accumulator 148 must be loaded in ascoderFreqParam.

The LSB of codePhaseParam represents 1/256th of a C/A code chip.CodePhaseParam initializes the contents of Phase Accumulator 148. Gen₋₋Enable is generated whenever Phase Accumulator 148 overflows. PhaseAccumulator 148 is a 25 bit register initialized by the value ofcodePhaseParam when corHoldRegLoad 152 from CPU 101 is active at each 1msec edge when new data is written from CPU 101. The 24 LSB's of 25-bitPhase Accumulator 148 are then added to coderFreqParam in Phase Adder150 and returned to Phase Accumulator 148. Phase Buffer Register 154stores and buffers the contents of Phase Accumulator 148, to produceCoderPNext which is updated whenever codCodeSAve 158 from gpsctl 182 isactive. CoderPNext is applied to Sat₋₋ Mem 186 for storage in RAM R2105. The operation of multiplexer 142 may be best understood from thedescription below of the triple multiplexing of ASIC 102 provided withFIGS. 10 and 11.

Gen₋₋ Enable is applied to Code Generator 138 to cause a new code to begenerated. C/A Codes parameters G1 and G2 are parallel loaded from RAMR2 105 by Sat₋₋ Mem 186 as g1ParIn and g2ParIn into Code Generator 138to produce g1GenOut and g2GenOut which are returned to RAM R2 105 bySat₋₋ Mem 186. The bit-0 of both G1 and G2 generators in Code Generator138 are internally XORed and generate genSerOut 160 which is seriallyapplied to 11 bit Code Shift Register 170 in Correlator Block 110, asshown in FIG. 5. Code Generator 138 generates the following C/A codes:

    G1=1+X3+X10

    G2=1+X2+X3+X6+X8+X9+X10.

The output of Code Shift Register 170 is applied to correlators 74, 11bits at a time at 48f₀ so that at least 20 code delays, separated by onehalf chip width, are correlated against each Doppler rotated sample fromeach SV. The increase in chip rate from 2f₀ to 48f₀ provides amagnification factor of 24 as will be described below in greater detail.

Values of G1 and G2 are be stored in RAM R2 105 during each segmentafter correlation with the doppler rotated sample in correlators 74 forthat SV so that they may then be retrieved by Coder Block 112 during thenext time segment for correlation of the next 11 bit sample from thesame SV.

Referring now also to FIG. 8, Correlator Block 110 is shown in greaterdetail. DopIOut and dopQOut in the rotated SV output from Doppler Block108 are applied to serial to parallel converter 166 which is thenparallel loaded to Holding Register 140. GenSerOut 160 from Coder Block112 is applied to Code Shift Register 170 in Correlator Block 110. Thesedata sets represent the doppler shifted data received from the SV, aswell as the locally generated code for that SV, and are applied toExclusive NOR gate correlator 74 for correlation under control of gpsCtl182.

The output of correlator 74 is applied to Adder 174 and combined in BitCombiner 176 to corIOut 178 and corQOut 180 which are applied to IQACCUMBlock 114 and IQSQACCUM 116 shown in FIG. 5. Adder 174 and Bit Combiner176 operate as a partial accumulator as indicated by accumulator 175 inFIG. 5.

Referring now also to FIG. 9, on overview of the operation of ASIC 102is shown. A dedicated set of on-chip logic controls the operation ofASIC 102 and is identified herein as gpsctl 182. In particular, underthe control of gpsCtl 182, sample data 100 from the GPS satellites isapplied to CACAPT 104 where it is separated and decimated into I and Qdata streams to form CACAPT Data output 123. SV data 123 is rotated forthe predicted Doppler shift of each SV to produce rotated SV outputsignals dopIOut and dopQOut which are correlated with genSerOut 160 fromCoder Block 112 in correlators 74. CorIOut 178 and corQOut 180 fromcorrelators 74 are accumulated in IQACCUM Block 114 and IQSQACCUM 116 toproduce output 184 to CPU 101.

As will be further described below in greater detail, a portion ofmemory is used for Sat₋₋ Mem 186 which stores and provides the dopplershift and code information required during multiplexing.

In operation, every millisecond is divided into 186 segments, each ofwhich includes 264 clocks. Within these 264 clocks, 12 channels areprocessed with each channel taking 22 clocks to compute 22 differentcorrelations or delays. Only 20 of these 22 correlations are stored andused for subsequent processing. For each channel, gpsCtl 182 controlsthe loading of Carrier₋₋ NCO 125 in Doppler Block 108 using dopLoad anddopSave. Similarly, gpsctl 182 controls the loading of Coder₋₋ NCO 136in Coder Block 112 via corHoldRegLoad and corCodeSave. The flow of datathrough Correlator Block 110 is controlled with serialShiftClk, and alsocorHoldRegLoad and codCodeSave. Control signals are applied to IQACCUMBlock 114 and IQSQACCUM 116 for each channel and include startSegment,startChan, resetAcc, peak, iqsq, wrchan, ShiftSelIqSq and acq₋₋ mode.Within each segment, gpsCtl 182 provides the periodic signals eng₋₋capShiftClk, capLoad, syncpulse, serialShiftClk to CACAPT 104 torepackage incoming satellite data samples into groups of 11 half chipsamples.

All accesses initiated by gpsCtl 182 are processed by Sat₋₋ Mem 186 togenerate read/write control and address signals for RAM R1 103 and RAMR2 105. GpsCtl 182 controls the flow of data through all data pathstogether with Sat₋₋ Mem 186 and manages the access of channel parametersstored in RAM R1 103 and RAM R2 105. RAM R1 103 is written to by theuser to define the channel parameters that will be loaded to RAM R2 105at the end of the corresponding integration or accumulation time. RAM R2105 is used by the datapath as a scratchpad to store the intermediatevalues of the various channel parameters during processing.

Data read out of RAM R2 105 is sent to the various parameter registersin Doppler Block 108, Coder Block 112, Correlator Block 110 and gpsctl182 under the control of Sat₋₋ Mem 186. Data from these blocks and RAM1190 are multiplexed at the input to the write port of RAM R2 105. RAM R1103 is a 16×108 asynchronous dual port ram used for the parameters forall 12 channels while RAM2 192 is another 16×108 asynchronous dual portram used for storing intermediate values of the satellite parametersduring processing, while switching from one channel to the next.

Referring now to FIG. 10, the system of the present invention includes amultiplexed data path in order to reduce the size and complexity of ASIC102 on which the majority of the parts of the system can be provided.Conventional receiver designs have multiplexed a single set ofcorrelators for use for each of the separate channels in which an SV istracked in order to reduce the number of correlators required. The useof the system of the present invention reduces the million or more gatesthat would be required for a conventional configuration down to amanageable number, on the order of about less than 100,000.

In accordance with the present invention, in addition to multiplexingthe satellite channels in a manner in which no data is lost, the codedelay correlations are also multiplexed. That is, conventional receiversuse two or three correlators to provide early, late and/or promptcorrelations for each SV. The present invention multiplexes a pluralityof code delays in order to provide far more code delay correlations thanhave been available in conventional systems without substantiallymultiplying the hardware, or chip area on ASIC 102 required by thenumber of gates used.

The multiplexing of code delays permits the wide capture windowdescribed above with regard to FIGS. 3 and 4 that permits rapid SVreacquisition. In particular, 20 delays such as 1/2 chip delays areprovided and constantly monitored for each SV so that GPS data can beacquired even during brief glimpses of the SV, for example, when car 10is in intersection 22 as shown in FIG. 1. The SV can be reacquired anduseful data obtained because the modelling of the vehicle's position onroadway 12 is sufficiently accurate to keep the predicted code anddoppler values for a previous acquired and currently obscured SV withina window of ±10 half chip code delays. In this way, data obtained duringreacquisition can be used directly as GPS data. That is, thereacquisition mode is transparent to the tracking mode. The GPS data isacquired whenever available without substantial lost time forreacquisition.

Further, the operation of satellite tracking is itself multiplexed foreach set of data for all 12 channels in order to further substantiallyreduce the ASIC gate count. That is, only a small portion of the bits inthe C/A code is processed at one time for all 12 SV's. In order todigitally process the signals received, the digital representations ofthese signals must be processed in registers and buffers capable ofstoring the digital data. The C/A code contains 1023 bits in eachrepetition which lasts 1 msec. If all 1023 bits were to be processed atonce, registers 1023 bits wide would be required. Such registers wouldbe expensive in cost and gate count and quite cumbersome. In accordancewith the third level of multiplexing used in the triply multiplexedreceiver configuration of the present invention, a smaller register ismultiplexed to handle different portions of the 1023 bits of the C/Acode. This means the smaller register is used many times during each 1msec repetition of the C/A code to process enough smaller samples of thedata received so that within each msec all 1023 bits can be processed.

In the preferred embodiment described above particularly in FIGS. 3 to9, a configuration using 11 bits registers was used so that eachregister is used 186 times per msec to process all 1023 bits of a C/Acode repetition. Each 1/186th of a msec is called a segment. Thetracking of each SV is therefore multiplexed 186 times by processing the11 bits in each register during each segment. In addition, in thepreferred embodiment, 12 channels are used to track a maximum of 12SV's. This requires that each 11 bit segment is multiplexed 12 timesduring that segment to apply a doppler rotation for each SV.

Further, each channel is further multiplexed by a factor of 22 toprovide a substantial plurality of different code delays. This requiresthat the doppler rotated sample for each SV is correlated 22 times withdifferent C/A Code delays before the doppler rotated sample for the nextchannel is produced. In this manner, 22 different code phases may betested for each of 12 SV during each of 186 segments to provide realtime data with only 11 bit wide registers by processing each register186 times per msec.

It is important to note that the processing of the present inventionoccurs during a particular segment, i.e. a 1/186th of a repetition ofthe C/A code, during the length of time required for the segment to becollected. In this optimized manner, no data is lost during tracking orreacquisition or switching between these states because the data beingprocessed in any particular segment is at most 11 half chips delays old.

Referring now to FIGS. 10 and 11, the output of Digital Filter 118 shownin FIG. 5 is sample data stream 119 at 2f₀. The chip rate of the C/Amodulation of the signals 100 from the SVs is at f₀. In order to avoidloss of any data, the SV signals must be sampled at least at theirNyquist rate, that is, at twice the chip rate of the modulation ofinterest which is 2f₀. Although sample data stream 119 can be operatedat a higher chip rate than the Nyquist rate, which is twice the chiprate, there is no advantage in doing so.

Sample data stream 119 is therefore a series of samples of the digitizedand filtered SV data at twice the chip rate of the C/A code, that is,each sample in sample data stream 119 has a width equal to one half of aC/A code chip. The number of bits in each msec or cycle of code insample data stream 119 is twice the number of bits in the modulation,i.e. 2046 bits each representing one half of a C/A code chip. Inaccordance with the multiplexing scheme of the preferred embodimentbeing disclosed, the data is processed in 11 bit segments, and sampledata stream 119 is therefore applied serially to 11 bit (10:0) registervalue buffer 120. The time required to serially store 11 bits out of atotal of 2046 bits in the 2f₀ data stream is 1÷(2046÷11=186) or 1/186thof a msec.

During the time the first set of 11 sample bits are being stored in 11sample deep buffer 120, no bits are available for processing. After thefirst 11 sample bits are serially received and serially stored, the 11sample bits are transferred in parallel to parallel block 122. Thisparallel operation therefore occurs every 1/186th of a msec or at a rateof approximately 0.18f₀. Each 1/186th of a msec is called a time segmentor segment and is the unit of processing for most of the operations. The1023 chip C/A code of each of the satellites in the composite signalreceived is processed in 11 half chip bits. Dividing the msec repetitionrate of the C/A code into 186 time segments multiplexes each of the 11bit registers by a multiplexing factor of 186.

CACAPT Data output 123 from parallel block 122 is processed in DopplerBlock 108 at a much faster chip rate, for example at 24f₀. That is, the11 bits of sample data in each segment of time is multiplexed by afactor of 12 to permit 12 different operations to be performed to thatset of 11 bits of data. In particular, in Doppler Block 108, CapIOut andCapQOut of CACAPT Data output 123 are multiplied in Doppler Register 124by twelve different doppler shifts so that within each segment twelvedifferent doppler rotations are performed.

Each different doppler shift represents the predicted doppler rotationrequired for each of the maximum of 12 different Svs that can betracked. The increase in processing chip rate from 2f₀ to 24f₀multiplexes the processing for each of 12 channels of data. It isimportant to note that the multiplexing to permit one channel to operateas 12 multiplexed or virtual channels each representing a different SVis applied only after the input signals are multiplexed, that is, brokeninto 186 time segments each including 11 half chip width bits. In thisway, the multiplexing for 12 channels or satellites is easilyaccomplished with relatively inexpensive 11 bit registers without lossof time or data. The selection of the number of sampling to be aninteger division of the number of code bits per period is important toachieve these goals. Multiplexer Block 129 in Carrier₋₋ NCO 125 controlsthe timing of this multiplexing under the direction of gpsctl 182.

The output of Doppler Block 108, signals dopIOut and dopQOut, areapplied to serial to parallel converter 166 within Correlator Block 110.Each rotated SV output signal 127 represents the rotated signal from asingle SV and 12 such rotated SV output signals 127 are produced in eachsegment of time.

Rotated SV output signal 127 is loaded in parallel fashion into HoldingRegister 140 in Correlator Block 110. The input to Exclusive NOR gatecorrelator 74 is therefore an 11 bit wide signal which is retained for1/12th of a time segment as one input to Exclusive NOR gate correlator74.

Correlator 74 is a series of 11 separate one bit correlators which alloperate in parallel. One input is rotated SV output signal 127 while theother 11 bit input is provided by 11 one bit genSerOut 160 output bitsfrom Coder Block 112. During the 1/12 of a time segment provided foroperation on the rotated SV output signal 127 for a particularsatellite, the code for that SV is produced serially by Code Generator138 and applied to Code Shift Register 170.

At the beginning of the correlation for a particular channel, 11 bits ofthe code for that SV have been shifted into Code Shift Register 170 andare available therein for correlation. Every 1/22nd of a channel (thatis, a 1/12 of a segment) each of the 11 bits in Code Shift Register 170are correlated in one of 11 one bit exclusive Nor gates in Exclusive NORgate correlator 74. This produces 11 correlator output bits, the sum ofwhich indicates the magnitude of the correlation between the rotated SVoutput signal 127 and that code phase. These 11 correlation sumsproduced in parallel are summed in parallel and stored in the first of22 summers related to that SV in Accumulator Block 115.

During the next or second 1/22nd of a channel, Code Generator 138produces the next bit for the C/A code for that SV. This next bit isapplied serially to Code Shift Register 170. At this time, 10 bits fromthe first correlation remain in Code Shift Register 170 and togetherwith the newest bit form another 11 bit sample of the expected code forthat SV, delayed from the previous 11 bit sample by the time required togenerate 1 bit, that is, one half chip width at the rate code isproduced, 48f₀. The second sample is therefore a one half chip delayedversion of the code, delayed one half chip width from the previous 11bit samples. It is important to note that the two 11 bit code samplesjust described differ only in that a new bit was shifted in at one endof the register to shift out the MSB at the other end of the register.

The 11 bit correlation product of the same rotated SV output signal 127and the second 11 bit sample of code is then stored in the second of the22 summers related to that SV in Accumulator Block 115. Thereafter, theremaining 20 serial shifts of the genSerOut 160 from Code Generator 138are correlated against the same rotated SV output signal 127 to produce20 more sums of 11 bit correlations for storage in Accumulator Block 115for that SV. The result is that 22 values are then available withinAccumulator Block 115 for processing, each value is a measure of thecorrelation of the signals from one SV with 22 different code phases ordelays, each separated by one half chip width.

During the next 1/12 of a time segment, that is, during the processingof the second multiplexed channel, the rotated SV output signal 127 forthe next SV, is applied to Holding Register 140 for correlation with 22different one half chip delays of the code generated for that satellite.At the end of a segment, Accumulator Block 115 includes a matrix of 12by 20 different sums. In one implementation of the present invention, ithas been found to be convenient to save only 20 out of the 22 possiblecode delay correlation results. The 12 rows of 20 sums represents themeasure of correlation for each of the 12 SV's at 20 code phases ordelays.

In summary, the data path for the present invention is triplymultiplexed in that

(a) each msec, which represents 1023 bits of C/A code, is sliced into186 to form the 186 segments in a msec of sample so that only 11 halfchip wide sample bits are processed at one time;

(b) each segment is then multiplexed by 12 so that each such 11 bitsample is rotated for twelve different sources;

(c) the rotated 11 bit sample for each source is correlated against 20sets of different code delays for that source to multiplex within eachchannel by 20; and

(d) the sum of the correlation products for each delay in each channelare then summed to produce the accumulated correlation output.

Although 22 different delays are available, it is convenient to use 20such delays, or code phase theories for testing the rotated satellitesignal. The correlation product having the greatest magnitude for eachchannel after accumulation, that is, the largest of the 20 sums of 11bits stored in Accumulator Block 115 for each channel may then bedetected by its magnitude, for example by a peak detector, to determinewhich delay theory is the most accurate. The peak sum represents theon-time or prompt correlation for that SV.

Turning now specifically to FIG. 11, the triple multiplexing scheme ofthe present invention may easily be understood by looking at the slicesof time resulting from each of the multiplexing operations. Within eachmsec, the C/A code for each particular satellite has 1023 bits. In orderto preserve all necessary information, the satellite signals aresampled, in a digital composite of signals from all satellites, at theNyquist rate at 2f₀ to produce 2046 half chip wide sample bits.

Each sequential set of eleven sample bits are processed together as asegment of time, the length of which is equal to 1/(2046÷11) of a msec,i.e. one 1/186th of a msec. After processing of the 186th segment in amsec all necessary data has been extracted and the 11 bit sample for thenext segment is available. Although the partial sums accumulated overeach msec in Accumulator Block 115 may only be evaluated at the end of amsec, no data is lost and the results are only 1 segment late. That is,since it takes 1 segment to fill 11 sample deep buffer 120 and transferthe 11 bit sample to parallel block 122, the data from the first 11 bitsample is being processed while the data for the second 11 bit sample isbeing collected. Even if the system operated for a year, the sampledbeing processed to provide position information is still only one timesegment old.

The 11 bits of each segment are multiplexed for each SV by being timedivision multiplexed during doppler rotation. That is, the 11 bit sampleof segment 1 is used to provide 12 different doppler shifted outputs sothat a single 11 bit segment sample is used 12 times to produce 12different satellite specific doppler rotated versions, assuming all 12satellites are in view or being modeled. The operations for one channelthen require one twelfth of a segment. It is critical to note that eachsegment only produces a partial result and that the 12 partial resultsduring each segment must be summed at the end of each msec to providevalid output data.

Each of the operations on one particular channel in a segment are timedivision multiplexed by a factor of 22 so that 22 different code delaysfor that partial sum for that satellite can be tested. The peak sum ofthese 22 correlations can however be detected by magnitude immediatelyif necessary to select the most likely delay for that channel. In thepresent embodiment, the information for that channel is only valid onceper msec when summed or accumulated so that there may not be asubstantial advantage in peak detected with a particular segment. Insome GPS applications and in other spread spectrum applications, such aswireless communications, it may be desirable if strong signals arepresent to accumulate and transfer the sum of the accumulations for eachsource from R3 to R4 more often than once per code repetition rate. Thetime required to evaluate a particular code phase delay or theory isonly 1/22nd of the time required per channel per segment or 1/22nd of1/12 of 1/186th of a msec. This speed of operation is more easilyachieved because the 11 one bit correlations required are produced inparallel. Similarly, the speed of generation of the different codedelays for a particular SV is more easily accomplished in accordancewith the present invention because each 11 bit code delay sample isautomatically produced when each single new bit, i.e. each new genSerOut160, is shifted into Code Shift Register 170.

The selection of the magnitudes or multiplexing factors used in eachlevel of multiplexing is not arbitrary. The larger the number ofsegments, the smaller the required size or depth of the registers needfor each sample. By using a code repetition multiplexing factor of 186,that is, by dividing the 2046 bits of a 2f₀ by 186, only 11 sample bit,need to be evaluated at a time.

The number of required channels is bounded pragmatically by the factthat at least 4 Svs must be in view at the same time to determineposition accurately in three dimensions. Time is the fourth unknownwhich must be determined along with each of the three dimensionsalthough provisions for estimating, modeling and/or updating theposition information as described above so that position information maybe accurately provided even during periods when less than 4 satellitesare concurrently in view.

The constellation of 24 NAVSTAR satellites in use are arranged to coverthe earth so that a maximum of 12 such satellites may be in view at anyone location at any particular time. The maximum number of pragmaticallyuseful channels is, for this reason, no less than about 12 channels. Theselected channel multiplexing factor used in the channel level ofmultiplexing in the embodiment shown herein is therefore a factor of 12.

The number of different code delays is bounded at the low end by anabsolute minimum of 1 so that if the exact delay can somehow bemaintained, the only necessary correlation would be the on-time orprompt correlation. Conventional GPS receiver systems use at least 2 or3 different code delays so that conventional tracking techniques, forexample those which use early, prompt and late correlations to centerthe prompt correlation within ±1 delay, may be employed.

In accordance with the present invention, a substantially greater numberof different code delays, or delay theories, are tested so that fastreacquisition may be accomplished as described above with regard toFIGS. 3 and 4. Although for the particular preferred embodimentdescribed herein, it was determined that a total of 20 different delays,each separated in time by one half the width of a C/A code chip, i.e.1/2 of 1/2046 of one msec, a code delay multiplexing factor of 22 wasselected because the relationship between each of the 3 multiplexingfactors is also important.

The product of the three multiplexing factors, code repetitionmultiplexing factor, channel multiplexing factor and code delaymultiplexing factor should optimally be an even integer multiple of thenumber of bits in each repetition of the spread spectrum modulation. Aneven integer multiple is required because samples must be taken at twicethe chip rate, i.e. at the Nyquist rate, in order to avoid data lossfrom sampling at a slower rate. Although multiplexing factors can beused successfully even if the product is not exactly equal to an eveninteger multiple, data loss or unnecessary complexity and costs mayresult.

In the particular embodiment shown, the spread spectrum code of interestis the C/A code, each repetition of which includes 1023 bits. Inaccordance with the triple multiplexing product rule discussed above,the product of the three multiplexing factors must equal an even integermultiple of 1023, such as 2046. In the described embodiment, the coderepetition multiplexing factor is 186, the channel multiplexing factoris 12 and the code delay multiplexing factor is 22. The product of 186multiplied by 12 and then by 22 is 49104 which, when divided by 1023,equals 48. 48 is an even integer and therefore the particular set ofmultiplexing factors used in the present invention provides one ofseveral optimized systems.

The reason this multiplexing factor product rule works well in atrilevel multiplexing configuration for C/A code is that there are threeprime factors in 1023. That is, 1023 is the product of three primenumbers, 31, 11 and 3. Each of the three multiplexing factors is evenlydivisible by one of these prime numbers. For example, 186 is divisibleby 31 six times, 12 is divisible by 3 four times and 22 is divisible by11 twice.

Using each prime factor of the number of bits in the sampled bit rate inone of the multiplexing factors yields two or more different families ofmultiplexing configurations for C/A code spread spectrum receivers. Inthe first family, if 11 channels are desired, then either the coderepetition multiplexing factor or the channel multiplexing factor wouldhave to be divisible by 31. Although it may be desirable in certainapplications to use 31 or 62 different code delays, there is asubstantial advantage in making the code repetition multiplexing factoras large as possible. This reduces the number of bits required to besaved and processed in each segment. By selecting the code repetitionmultiplexing factor to be a multiple of 31, the number of delaysactually used can be more easily controlled because the code delaymultiplexing factor could be any multiple of 3.

In the other convenient family, 6, 9, 12, 15 or 18 satellite channelsare desired so that the channel multiplexing factor is an integralmultiple of 3. This permits the code delay multiplexing factor to be afactor of 11 while the code repetition multiplexing factor is a factorof 31. The particular embodiment described in the specification above isin this family.

Another constraint on the selection of multiplexing factors is the speedof operation of the lowest level of multiplexing. In the embodimentdisclosed, the third level of multiplexing operates at 48f₀. The clockspeed of the hardware implementation must be sufficient to permitoperation at this speed. As faster and faster on chip components aredeveloped, higher clock speeds may be used to accomplish the highestspeed processing and larger multiples may be used. For example, withcomponents in the high speed processing sections such as CorrelatorBlock 110 capable of operation at higher rates at multiples of f₀, suchas at 96f₀, the code repetition multiplexing factor could be doubled toproduce 24 channels with 20 delays or taps or 12 channels with 40 delaysor taps or 11 channels with 6 bits and 22 taps.

The system configuration may also be viewed from the standpoint of atime or speed magnification. Operation at the third multiplexing levelat 48f₀ is 24 times faster than the chip rate of the 2f₀ sample beingprocessed. This amplification factor of 24 permits a hardwaremultiplexing or gate compression factor of 24. The number of gates onASIC 102, or other devices for implementation the present invention, isreduced essentially in direct proportion to the magnification factor.All other factors being equal, the surface area of a chip operated at48f₀ is on the order of 1/24th of the surface area that would berequired to operate at 2f₀. Similarly, an increase in the magnificationfactor to 96 would permit a reduction in the required chip surface realestate required on the order of almost half.

The particular embodiment of the multiple level multiplexing spreadspectrum receiver of the present invention which has been disclosedabove is a GPS receiver. The same invention can be used for other spreadspectrum signals such as wireless telephone signals with dueconsideration for the selections of multiplexing factors based on thebit rate of the spread spectrum code used and the environmental factorsapplicable to that application. The environmental factors for thepresent configuration, such as the pragmatic constraints on the numberof channels and code phases, have been described above.

Having now described the invention in accordance with the requirementsof the patent statutes, those skilled in this art will understand how tomake changes and modifications in the present invention to meet theirspecific requirements or conditions. Such changes and modifications maybe made without departing from the scope and spirit of the invention asset forth in the following claims.

What is claimed is:
 1. A method of tracking a spread spectrum source,comprising the steps of:acquiring the source from a composite of signalsreceived by determining a delay parameter for that source related totime of receipt of a source specific spread spectrum code in the signalsfrom the source; correlating signals received from the source with aprompt version of a locally generated source specific code based on thedelay parameter; tracking the signals received from the source byupdating the delay parameter in response to the signals from the source;modeling the delay parameter to continue updating the delay parameterwhen signals from the source are not being tracked; and continuouslycorrelating the composite of signals received with a plurality ofdelayed versions of the locally generated source specific code based onthe modeled delay parameter to reacquire the signals from the source. 2.The method of claim 1 wherein the step of correlating the signals fromthe source further comprises the step of:correlating the signalsreceived from the source with an early and a late version of the locallygenerated source specific code based on the delay parameter to adjustthe delay parameter; and the step of continuously correlating thecomposite of signals received further comprises the step of: correlatingthe composite of signals received with a plurality of versions of thelocally generated source specific code different from the early or lateversion used.
 3. The method of claim 2 wherein the early and lateversions of the code are each separated from the prompt version by amultiple of a fixed delay.
 4. The method of claim 3 wherein each of theplurality of versions of the locally generated source specific code areseparated from the prompt version by multiples of the fixed delaydifferent than the multiple used for the early and late versions.
 5. Themethod of claim 4 wherein the fixed delay is related to the chip widthof chips in the source specific code.
 6. The method of claim 5 used fortracking GPS satellites, wherein the fixed delay is a fraction of a C/Acode chip.
 7. A spread spectrum receiver, comprising:means for providinga plurality of versions of a locally generated signal related to aspread spectrum signal to be received; means for combining at least twoof the versions of the locally generated signal with the spread spectrumsignal to produce a product signal related to each of the at least twoversions; means for evaluating the at least two product signals toadjust a parameter of a third version of the local signal; means forcombining the adjusted third version of the local signal with the spreadspectrum signal to produce a data signal; means for determining apredicted value of the parameter when the spread spectrum signal becomesunavailable; means for combining an additional plurality of versions ofthe locally generated signal related to the predicted value withreceived signals to produce additional product signals related to eachof the additional plurality of versions of the locally generated signal;and means for evaluating the additional product signals to produce areacquired data signal.
 8. The invention of claim 7 for use with GPSsatellites, wherein:the spread spectrum signal is a composite of signalsreceived from a plurality of GPS satellite; the locally generated codeincludes satellite specific C/A codes; the third version of the localsignal is a prompt version of the code; and the additional plurality ofversions of the satellite specific C/A codes are separated from theprompt version by positive and negative multiples of a fixed delay. 9.The invention of claim 8 wherein the separation in time from the mostpositive multiple to the most negative multiples of the C/A codesprovide a capture window in time sufficient to include accumulatedmodeling errors accrued during motion of the receiver.
 10. The inventionof claim 7 wherein the time required to reacquire is sufficiently shortto take advantage of gaps in obstructions causing unavailability of thesignals from satellites resulting from the relative motion the receiverand the obstructions.
 11. A method of operating a receiver for coded GPSsignals from satellites comprising the steps of:correlating early,prompt and late versions of a locally generated model of the code withsignals received from GPS satellites to adjust a delay of the promptversion to track a selected satellite; maintaining a predicted value ofthe delay when the selected satellite is unavailable; correlating aplurality of different early versions of the locally generated code,early with respect to the predicted value of the delay, with signalsreceived from satellites to produce correlation products; correlating aplurality of different late versions of the locally generated code, latewith respect to the predicted value of the delay, with signals receivedfrom satellites to produce correlation products; and reacquiring theprevious unavailable selected satellite by selecting the versionproducing the largest correlation product above a predeterminedthreshold.